The present invention generally relates to apparatuses for measuring the thickness of films, and more particularly to a film thickness measuring apparatus which is designed to easily measure the thickness of a transparent film provided on a transparent substrate.
Conventionally, a general film thickness measuring apparatus which uses the polarization analysis method as the operating principle thereof, impinges light from a light source on a measuring plate with an arbitrarily selected incident angle. The measuring plate is formed with a transparent film on top of a transparent substrate, and the thickness of the transparent film is to be measured. The light reflected from the measuring plate is detected in a detector of the film thickness measuring apparatus, and a detection output of the detector is analyzed in an analyzer system so as to measure the film thickness of the transparent film.
In this type of a film thickness measuring apparatus, the film thickness is not measured immediately. First, an amplitude ratio tan .psi. and a phase difference .DELTA. between two mutually perpendicular polarized light components of the reflected light which is obtained when the incident light is reflected by the measuring plate, are measured. On the other hand, different values for the film thickness are successively substituted into a predetermined equation which has a film thickness d as the parameter and will be described later on in the specification, so as to obtain the amplitude ratio and the phase difference between the two polarized light components by calculation. It is assumed that the value for the film thickness d which is substituted into the predetermined equation is the thickness of the transparent film which is measured, when the calculated amplitude ratio and the calculated phase difference are equal to the measured amplitude ratio and the measured phase difference, respectively (with an error within a tolerance).
However, according to the conventional film thickness measuring apparatus, when the amplitude ratio and the phase difference are taken along the X and Y coordinates and the film thickness obtained with respect to the amplitude ratio and the phase difference is plotted, the collection of the plots form an oval shape as will be described later in conjunction with a drawing. Thus, as may be readily understood, two values for the film thickness exists with respect to one phase difference, for example. For this reason, the film thickness cannot be obtained solely from the phase difference, and the film thickness must always be obtained from the amplitude ratio and the phase difference. Further, even when the amplitude ratio and the phase difference changes slightly in value, the value of the film thickness which is obtained changes greatly. As a result, there are disadvantages in that the film thickness measuring accuracy is poor, and that it takes a considerably long time to perform calculations and the like for obtaining the film thickness.